Synthesis of spinel LiNi0.5Mn1.5O4 cathode material with excellent cycle stability using urea-based sol–gel method

doi:10.1016/j.matlet.2012.08.126

Materials Letters

Volume 89, 15 December 2012, Pages 251-253

https://doi.org/10.1016/j.matlet.2012.08.126 Get rights and content

Abstract

The spinel LiNi_0.5Mn_1.5O₄ cathode material has been successfully prepared employing a simple and novel urea-based sol–gel method. The material exhibits excellent cycle stability to reach 132, 123 and 113 mA h g⁻¹ at 1, 3 and 5 C (25 °C), respectively. When cycled at 55 °C, 105 mA h g⁻¹ could be achieved at 5 C with only 8.6% capacity fading after 150 cycles. In addition, the satisfactory high rate (20 C) cycle performance at 25 °C has been shown to reach 97.8 mA h g⁻¹, retaining 87.5% after 200 cycles. The results indicate that urea should be a universal and low-cost chelating agent for the synthesis of LiNi_0.5Mn_1.5O₄ cathode material with excellent electrochemical properties.

Highlights

► Urea as a chelating agent was firstly introduced in spinel LiNi_0.5Mn_1.5O₄ system. ► The material exhibits excellent cycle stability at 55 °C (91.4% Q-retention after 150 cycles) at 5 C rate. ► It shows satisfactory high rate cycle performance at 20 C discharge rate (87.5% Q-retention after 200 cycles) at 25 °C.

Introduction

With the increased requirements of high power energy storage systems in hybrid electric vehicle (HEV) and electric vehicle (EV) applications, spinel LiNi_0.5Mn_1.5O₄ (LNMS) cathode material for lithium ion batteries has drawn considerable attention based on its high operating voltage (4.7 V), high theoretical capacity (147 mA h g⁻¹) and unique 3-dimensional Li⁺ diffusion channels. However, the undesired interfacial side reaction between high-voltage charged LiNi_0.5Mn_1.5O₄ and electrolyte, along with the inevitable presence of Li_xNi_1−xO as a second phase, provokes serious capacity fading during cycling, especially at high rate and elevated temperature (55 °C). To overcome these drawbacks, several approaches [1], [2], [3], [4], [5] have been proposed, including the optimization of synthesis conditions. Sol–gel technology is regarded as an effective synthesized route in homogeneously mixing all reagents at atomic or molecular level and carefully controlling the particle size in a narrow distribution. The selection of chelating agents such as citric acid [6], malic acid [7], ethylene glycol [8], cellulose, ascorbic acid and resorcinol/formaldehyde composite [9], is a key factor for the synthesis of LiNi_0.5Mn_1.5O₄ by the sol–gel method.

As a simple and low-cost organic compound, urea has been reported as precipitation agent [10], combustion agent [11] or shape-controlled agent [12], but few works on chelating agent have been reported in LiNi_0.5Mn_1.5O₄ system. Therefore, in this work, we have synthesized spinel LiNi_0.5Mn_1.5O₄ cathode material employing a novel urea-based sol–gel method for the first time. The cycle performances of LiNi_0.5Mn_1.5O₄ at different rates and elevated temperature were discussed in detail.

Section snippets

Experimental

The as-prepared LiNi_0.5Mn_1.5O₄ was synthesized via a simplified sol–gel method employing acetates as raw materials and urea as a chelating agent. Stoichiometric amounts of all acetates were dissolved in de-ionized water and stirred at 50 °C. The urea solution (the mole ratio of urea and all metal ions is 1:1) was added with continuous stirring. After stirring constantly for 5 h, the temperature was raised to 80 °C for the evaporation of water. The brown transparent gel was obtained in a vacuum

Results and discussion

Fig. 1(a) shows the XRD pattern of the as-prepared LiNi_0.5Mn_1.5O₄ material. All the diffraction peaks can be indexed based on a well-defined spinel cubic structure, along with minor traces of Li_xNi_1−xO impurity phase close to the characteristic peaks of (311), (400) and (440). The lattice parameter calculated by an MDI Jade Software is 8.171 Å, which is well consistent with the values reported previously [5], [8]. As the SEM image (inset) and the corresponding particle size histogram shown in

Conclusions

High pure spinel LiNi_0.5Mn_1.5O₄ has been prepared employing a simple and low-cost urea-based sol–gel method. The results demonstrate that the material with small particles exhibits excellent cycle stability at 55 °C and 94.3 mA h g⁻¹ can be achieved at 5 C after 150 cycles (capacity fading rate: 8.6%). Besides, the satisfactory high rate (20 C) cycle stability has been shown to reach 97.8 mA h g⁻¹ at 25 °C, retaining 87.5% after 200 cycles. Using the inexpensive urea as a chelating agent should be an

Acknowledgments

This work was supported by the National Natural Science Foundation of China (no. 20973124) and Ph.D. Programs Foundation of Ministry of Education of China (no. 20070056138).

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With the development of mobile electronic devices, electric vehicles and other high-power equipment, LNMO attracted more attention due to the high operation voltage (∼4.7 V), high theoretical capability (∼148 mAh g−1) and environmental friendliness [3]. Kinds of methods to synthesize LNMO have been reported, such as solid state reaction [4], co-precipitation [5], sol–gel method [6], etc. Compared with other traditional methods, the hydrothermal method was easy to control the morphology and particle size selectively.
Hierarchical LiNi_0.5Mn_1.5O₄ (LNMO) microspheres were prepared via a hydrothermal method followed by program controlled temperature calcination. The as-obtained LNMO microspheres (diameter: ∼5 μm) which were assembled with submicron nanorices particles (length: ∼500 nm) (R-LNMO) could combine the superiority of the both nanosized and microsized structure, enhancing rates performance. The result indicated that R-LNMO exhibited discharge capacities of 141 mAh g⁻¹ after 200 cycles at 2 C, together with the capacity retention of 95.2%. Besides, R-LNMO could deliver capacities of 147, 141, 137, 128, 120, 114 mAh g⁻¹ at rates of 0.1, 2, 5, 10, 15 and 20 C, showing prospectively superior rates performance.
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LiNi_0.5Mn_1.5O₄ (LNMO) cathode thin films were prepared on Pt-coated stainless steel (SS) substrates by radio-frequency magnetron sputtering. The layers were post-annealed at 500, 600 and 700 °C to study the impact of thermal treatment on the crystallization of the cathode material. As the annealing temperature increases, interdiffusion occurs between the substrate and the LNMO film, which degrades the electrochemical properties of the cathode film. In order to mitigate this problem, an In-Sn-O (ITO) interlayer was introduced between Pt and the SS substrate. The ITO film is electrochemically stable and acts as an effective diffusion barrier between the substrate and the LNMO layer, which results in improved electrochemical properties of the LNMO film for a lithium ion battery.
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Synthesis of spinel LiNi0.5Mn1.5O4 cathode material with excellent cycle stability using urea-based sol–gel method

Abstract

Highlights

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgments

Electrochim Acta

J Power Sources

Electrochem Commun

Mater Lett

J Power Sources

J Alloys Compd

Electrochim Acta

Synthesis of spinel LiNi_0.5Mn_1.5O₄ cathode material with excellent cycle stability using urea-based sol–gel method